MECHANICS AND USEFUL ARTS. 95 



COMPOSITION OF GLASS. 







It is commonly believed that alumina promotes devitrification, 

 but from the experiments of the late M. Pelouze it would seem 

 that a glass containing a considerable proportion of alumina is 

 more difficult to devitrify than one containing a very small propor- 

 tion, as all common glass does. He found that a very satisfactory 

 plate glass can be obtained by employing 350 parts of sand to 100 

 parts of sulphate of soda and 100 parts of chalk ; the proportion of 

 sand could not be usefully increased beyond this amount. This 

 glass was found to have a very low refractive power. He concludes 

 that magnesia must be carefully avoided in the manufacture of 

 glass. 



Lead-Thallium Glass. This has greater density and refracting 

 power than common lead-flint glass; 300 pure sand, 200 minium, 

 and 335 carbonate of thallium (instead of the usual 100 carbonate of 

 potassa), give a glass of density 4.325, index of refraction 1.71, 

 with a very slight yellowish tint. ISInstitut, 1866. 







UNINFLAMMABLE PAINT. 



M. J. B. Harris, of Kentucky, has invented a paint which con- 

 sists in the combination of the ordinary oil paints with calcined 

 schist, shale, or mineral coal, which is reduced to a chalky and pow- 

 dered condition and freed from all hard and gritty particles. For 

 white and red lead the following proportions may be used : 3 

 parts b} 7 weight of the calcined schist, shale, or coal, to 1 part of 

 the lead ; and for ochres and umbers, 4 parts of the calcined ma- 

 terial to 1 part of the ochre or umber. No turpentine is used as a 

 dryer, but japan, litharge, or dryers of a thin material should be 

 employed. This preparation will resist the action of flame, sparks, 

 and live coals, so that floors painted with it" will not be liable to 

 take fire from the falling upon them of burning coals ; roofs will 

 also be protected from sparks from chimneys, or other light burn- 

 ing substances carried by the wind from burning buildings. 



SUMMARY OF IMPROVEMENTS IN THE MECHANIC AND USEFUL 



ARTS. 



Uninflammable Fabrics. M. Kletzinsky proposes the following 

 mixture for rendering muslins and other li^ht fabrics uninflam- 

 mable. He takes equal weights of commercial sulphate of zinc, 

 sulphate of magnesia, and sal ammoniac, and pounds all together 

 in a mortar; he then adds 3 times the weight of ammonia alum. 

 The addition of the last produces a pasty mass, which must be 

 carefully dried at a gentle heat and then powdered. To make 

 fabrics uninflammable, 1 part of the above mixture is used with 

 2 parts of starch. It is cheaper than tungstate of soda. 



Hydraulic Cement. In Dingier' s " Polytechnic Journal" the fol- 

 lowing is given for making a cement completely impermeable to 

 water. 2 parts of cement (Roman or Portland), 1 part of ground 



